Portable Network Graphics, or PNG, is a common file format used for images, on the Internet and elsewhere. While PNGs give you a very high-quality image with a very small file size, they have their limitations—because it's a pixel format, images won't look good when stretched beyond a certain size. Windows Meta File, or WMF, is another, vector-based, format often used in clip art. While WMF files have their limitations as well (vector files cannot support gradient shades, and thus are somewhat artistically limited), in certain circumstances they're exactly what you need. WMF files do not use pixels, and hence can be zoomed-in indefinitely. Because the formats are so wildly different, software to convert between one and the other can be difficult to find—but it does exist, and is very easy to use.
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Download and install Xara Xtreme (see link in References). Xara is a multi-purpose graphics program that is released under a commercial license for Windows. A free, full-featured 30-day trial version is available, and the full program can be purchased for less than $100. There is a completely free, open-source version of Xara Xtreme for Linux as well, but the Linux variant is not quite up to date with the current Windows version—it may not be able to export to WMF format.
Run Xara. Click on "File" > "Open..." and select a PNG image of your choice. The program will open the file and display it in the central panel.
Click on "Utilities" > "Bitmap Tracer..." to open the Tracer window. This is a sub-program that allows you to convert ordinary images, such as PNGs, into vectors—the mathematical format used in WMF files. Click the "Trace" button to preview your new vector image. Adjust the sliders and repeat until the preview fits you desires, and then click "Insert." You will now see your converted image at full size. If you original image was a photograph, it will likely look very posterized—this is normal. Vector formats generally use as few colors as possible, to keep file size down.
Click "File" > "Export..." and choose "EMF/WMF" from the list of file types. Select a location and file name, and click "Export." Another window will appear asking whether to save as an Enhanced Meta File or Windows Meta File. Choose the latter and click "OK."